In the offshore oil industry a variety of ship-shaped vessels, semi-submersible vessels, barges, and bottom-supported gravity contact platforms are utilized for oil exploration and development drilling. Production platforms are usually secured to the sea floor by pilings and are thus permanent structures. In the offshore polar seas the environmental conditions are too hostile for much of the year for surface floating vessels and barges due to the existence and movement of ice features.
It has, therefore, been regarded as necessary to utilize mobile structures which can be positioned over particular drilling and/or production sites and can then be submerged into bottom-supported gravity contact with the sea floor. Such structures must then remain functional for their intended uses even when large ice features impinge on the outer surfaces. Ice features such as multi-year ridges embedded in multi-year floes can apply sufficient force so as to actually cause movement of some such structures from their established locations.
When drilling activities are to be carried out in benign sea areas, a wide variety of platform configurations and erection techniques can be employed since low wave heights and ice-free weather conditions prevail. The gravity contact platform structures utilized in polar seas can also be used in such areas such as those encountered in the polar latitudes.
For oil drilling and production in arctic conditions, such as those encountered in the Beaufort Sea, the Chuckchi Sea, or the Bering Sea, specialized structural designs must be employed. In many parts of the arctic latitudes, the seas are generally covered with ice from October through June. A land fast ice cover begins to form early in October and grows seaward during the winter and spring, reaching a maximum thickness of approximately 7 feet by May. Break-up usually begins in early June and continues throughout the remainder of the summer. This land fast ice consists of two distinct zones. The first zone extends outwardly from the sore to a water depth of about 33 feet. This zone consists of smooth first-year ice with thickness up to approximately 7 feet. The second zone covers a water depth range of 33 feet to about 66 feet and contains a number of first year pressure ridges. These pressure ridges are formed by break-up of a fault line and collision through movement of two ice floes toward the fault line position from opposite directions. This action then produces ice rubble which remelts and freezes to produce a pressure ridge of ice. Ice floes in the second zone move forward with wind and water currents and can impose very high impact forces on any stationary structure.
First year ice ridges which survive the summer period evolve into multi-year ice ridges which are more consolidated than first-year ridges. Both first-year ridges and multi-year ridges are found in the second zone.
Beyond the second zone and out to a position just past the continential Shelf, at water depths exeeding 200 feet, is the transition zone which contains large pressure ridges that move around in sporadic fashion. Beyond this zone is the permanent polar ice pack which is composed primarily of multi-year ice. This description of ice conditions holds particularly for the south part of the Beaufort Sea between Harrison Bay and Prodhue Bay.
Another environmental condition encountered in the Beaufort Sea area is that the sea floor consists mainly of pleistocene clays and holocene silts which have low force-bearing properties. As a result of this soft sea floor, some oil production equipment such as the well cap valve and tube systems known as "Christmas Trees" have been known to sink into and disappear in the silt unless adequately buoyed.
The combination of ice floes on the surface of the sea and soft, low force-bearing, sea floor soil conditions presents special problems for polar offshore structure design and operations. The first problem is that the ice floes impact any support member extending through the water surface and this, in turn, tends to push the entire platform off of its drilling location.
The force of large ice features has been found to be sufficient to move a large multi-ton platform. Due to the soft sea floor soil conditions, such platforms cannot be adequately secured to the sea floor by piles or other economically feasibly means.
One type of structure which has evolved for such conditions is shown in U.S. Pat. No. 4,080,796 to Edling et al. Three circular cross-section columns, are utilized to support the drilling platform above a buried mat. When the migrating ice features encounter such circular cross-section columns, the ice floe fails by crushing, buckling, and shearing of the impacting ice feature. The compressive or crushing strength of sea ice depends upon its structure, temperature, confinement, brine volume, strain-rate, and any flaws present; and generally ranges from about 200 to 1500 pounds per square inch. A structure of the type shown in this patent may be able to successfully resist the lateral forces imposed by first-year sheet ice and first-year pressure ridges in the first and second zones.
A problem of another order of magnitude however, is encountered when a large multi-year floe or multi-year ridge embedded in a multi-year floe impacts one or two of the support columns shown in U.S. Pat. No. 4,080,796. The resulting lateral force imposed on the structure is sufficient to move it off station and even to tilt it, particularly in view of the off-center placement of the drilling derrick. U.S. Pat. No. 4,314,776 to Palmer et al. also shows a structure designed to break surface ice floes by crushing and shearing fracturing.
It is also possible to utilize a third type of ice failure in order to break the ice features encountered in hostile actic environments. By configuring the structure to force the multi-year ice floe and multi-year ice ridge to bend upwardly away from the horizontal flotation plane, it is possible to fracture the floe and ice ridge by out-of-plane bending failure. Such a failure mode imposes large vertical forces on the portion of the structure which forces the multi-year ice ridge to move upwardly out of its flotation plane. While such out-of-plane bending or flexural failure results in flexural stress in the ice, the flexural strength is only about 80 to 130 pounds per square inch.
The large sizes of the multi-year ice ridges embedded in multi-year floes also impose horizontal forces on the structures in the 30,000 kips to about 200,000 kips range (1 kip=1,000 lbs.) These multi-year ridges have been the subject of considerable investigation in recent years. A typical large ridge may have a length of several thousand feet, a width of 335 feet, a total depth at the mid-point from sail to bottom of the keel of 85 feet, and be embedded in a multi-year ice floe having a thickness of up to about 28 feet. The keel width in the direction perpendicular to the ridge length is about 110 feet. The angle from the bottom of the keel to the bottom of the multi-year ice floe varies substantially and averages about 30 degrees.
In order to effect the break-up of such large ice features, a number of bottom-supported gravity contact structures have been evolved. The simplest forms of these structures are those having a single sloped surface to promote out-of-plane bending failure of large ice features such as shown in U.S. Pat. No. 3,952,527 to Vinieratos et al. Another structure of this type is shown in U.S. Pat. No. 3,793,840 to Mott et al.
Yet another type of structure is the multiple-surface type such as shown in U.K. Pat. Nos. 2,017,793, 2,017,794 and 2,018,700. In these structures a wide flange with an angle of about 15.degree. to the horizontal plane forms the base of the structure. A second sloped surface is positioned close into the center of the structure for effecting the crushing break-up of surface ice floes. In such structures the waterline diameter is kept as small as possible to reduce the total ice forces loading. The relative proportions of the base flange annular width compared to the structure total radius results in the multi-year ice ridges being fractured by out-of-plane bending independently of the break-up of the surface ice floe. The vertical forces imposed on structures if this configuration by such out-of-plane bending fracturing of pressure ice ridges are in the range of 50,000 to 200,000 kips.
U.S. Pat. No. 4,325,655 to Jahns et al. shows structures similar to the above U.K. patents in FIGS. 2-4. Other modifications which present the same ice fracturing mechanics are also set forth in which the fracturing of surface ice floes occurs separately from the break-up of the multi-year pressure ridges. Such separate fracturing mechanics are due to the relative proportioning of the ice imparting surfaces. The ratio of the lesser sloped wall section annular width to total structure radius varies from 0.35 to 0.7 in U.S. Pat. No. 4,325,644. This proportioning insures that the surface ice floes will be fractured separately from the pressure ridge fracturing which will, in turn, tend to reduce the peak ice loading on the structure. The ice impact walls are constructed of steel plate which is subject to permanent deformation at the ice loading conditions encountered. Such deformation is due to the malleability of the steel plate employed.
It is recognized in this art that the largest vertical forces imposed on the sloping surface structures occur just prior to the fracturing of the multi-year ice ridged.
Another factor in the constitution of these structures is that the sloped ice contacting flange must be supported by internal reinforcements in order to withstand the high impact force loads. When a steel-only fabrication design is used for these structures, engineering calculations show that the internal reinforcements have to be so closely spaced together that it is very difficult to perform the necessary welding to secure the reinforcements required.
U.S. Pat. No. 4,080,798 to Reusswig et al. shows a single sloping surface, bottom-resting drilling island which combats the ice floe problems by providing circulated heating fluids in the ice walls. U.S. Pat. No. 4,265,569 to Gefrert shows a structure having steeply sloping surfaces with a 45.degree. angle on the lower surface and a 60.degree. angle on the upper surface, and proposes to solve the ice impact problems by means of a heating source similar to that of the U.S. Pat. No. 4,080,798.
Thus, the multi-sloped arctic structures are characterized by sloped bottom flanges which extend inwardly for a large percentage of the radius of the overall structure and thus are the only part of the structure which is effective for causing bending fracturing of multi-year ice ridges. Another characteristic of these structures is that the ice contacting surfaces of both the base flange and the second sloping surfaces are fabricated by steel plate which then necessitates the use of reinforcements which cannot be economically constructed.
Another problem with the above described multi-sloped offshore construction is that these structures are all installed on the sea bottom without regard to the depth in which they are installed. There is no provision for elevating the structures above the sea floor in order to maintain the ice floe contacting level at a nearly equivalent vertical position independently of the water depth. The problem encountered is that placement of such structures in 40 foot depths encounters significantly different ice break-up mechanics than those encountered in 80 feet of water since the ice features then contact the sloped surfaces located at higher vertical positions on the structure. Other problems with these structures are that the on-loading and off-loading is rendered difficult since there is no water surface loading capacity. Another problem is that controlled positioning of the structures during installation is not provided for and hence tilt over and/or horizontal final resting positions can occur during installation.
Part of the above problems evolve from the mechanics of ice feature break-up by fixed structures. Surface ice floes are anistotropic and fracture through the modes of crushing, buckling, shearing and bending. The latter mode operates to extend fracture places radially outward from the fixed structure. Such planes weaken the ice floe and will continue to propagate through a given floe sheet when it collides with a vertical or near-vertical wall of the structure. This surface floe break-up then produce ice floe "ride-up" on the structure which can result in large ice loading forces of about 30 to 40 percent of the total ice load. This ice "ride-up" problem has resulted in marine engineers reducing the waterline diameters of the structure so that less ice rubble accumulates.
Another type of problem is encountered by the prior art structures when multi-year ice ridges embedded in multi-year floes are encountered. These large ice ridges come in contact with the structures far from the position of the sloped surface which penetrates the water and thus breaks up surface ice floes. The ice ridge is then wedged upon the incline plane of the first sloped surface and its entire prefracturing weight is taken up by the structure. Both vertical and horizontal forces are imposed on the structures during this impact. The ratio of the horizontal force to the vertical force depends upon the slope of the ice wall and the coefficient of friction at the ice/structure interface. The ratio decreases as the slope decreases and as the coefficient of friction decreases.
Ice ridges can be fractured more easily by out-of-plane vertical bending due to their geometry and strength characteristics. During the fracturing and clearing process a central and two outlying hinges are formed in the ridge. The large vertical forces of 50,000 to 200,000 kips encountered in this process can drive the structure base flange into the soft sea bottom. The opposite side of the structure base then breaks loose from the bottom and the structure can slide or come to rest at a tilting angle. Such displacement problems are severe when drilling an oil well.
The provision for fracturing large ice features with lower vertical forces and lower horizontal forces imposed upon the structure would be a large step forward in the offshore industry.